The field of the disclosure relates generally to gas turbine engines and, more particularly, to a system and method of supplying a purge fluid to a cavity of a fuel supply circuit.
At least some known gas turbines include a compressor, one or more combustion chambers, and an expansion turbine. The combustion chambers are supplied with gaseous fuel through a fuel supply system to be mixed therein with pressurized relatively high temperature air from the compressor. The fuel supply system permits several types of fuels, for example, natural gas, liquid fuel or synthetic gas or “syngas” to be supplied to the combustion chambers. The fuel supply system permits regulating a plurality of fuel supply system parameters from the fuel source to the one or more combustion chambers. Specifically, fuel supply systems typically permit regulating the fuel pressure, fuel temperature and fuel flow to the one or more combustion chambers.
To permit routing and transfer of various types of fuels to the gas turbine, ensure regulation of pressure, temperature and flow conditions, the supply circuit includes isolation valves, flow regulation valves, and cooling and filtration systems.
Furthermore, the fuel supply system must be capable of ensuring separation of portions of the fuel supply system, for example, of cavities, to avoid contact between relatively high temperature air and the gaseous fuel sources to prevent self-ignition of the fuel and creation of explosive mixtures. Typically, a gas turbine is operated on one of two types of fuels. For example, the first fuel is natural gas and the second fuel is synthetic gas or syngas, each fuel supplied through a separate circuit for at least a portion of the fuel supply system. Each fuel circuit, when not in use, may be purged with relatively high temperature air extracted from the turbine compressor. To isolate each fuel circuit from this relatively high temperature air, a cavity sometimes referred to as a “double block and bleed” valve arrangement is generally used. When the valves of the “block and bleed” valve arrangement are incorporated in a single component, the single component is referred to as a block and bleed manifold. After using a fuel supply circuit, it is purged with an inert gas, for example, nitrogen, before introduction of relatively high temperature scavenging air to avoid creating an undesirable mixture.
For example, if the second fuel supply circuit is isolated during a securing of the gas turbine or a change of fuel from the second fuel to the first fuel, no fuel circulates in the second fuel circuit and the supply circuit of the second fuel is purged with relatively high temperature air. During this purge phase, a flow of relatively high temperature air is maintained towards the passages in the injectors provided for the second fuel to avoid condensation, burning the nozzle tip, and limit the risk of a return of gas from the combustion chamber to the second fuel supply circuit.
When the valves controlling the fuel or relatively high temperature air supply for purging are closed, there still is a risk that, in some cases, of fuel gas leakage into the dead leg cavities formed by the isolation valves and thus causing a risk of contact between the relatively high temperature purge air, whose temperature may attain 500° C., and the fuel.
Known solutions using inert gases to ensure separation between the fuel and relatively high temperature purge air include filling a cavity between two isolation valves with inert gas at a predetermined pressure and maintaining an appropriate pressure to compensate for any fuel pressure and relatively high temperature purge air pressure variations.
The inert gas pressure level is selected based on an expected maximum fuel pressure or relatively high temperature purge air pressure. Maintaining a high pressure of inert gas in the cavity may require an expensive compression system and may also consume a large quantity of inert gas to make-up for leakage through, for example, the isolation valves. Maintaining a high pressure of inert gas in the cavity may also be influenced by other factors, such as ambient temperature and turbine load level. This technique also imposes the need for storage of inert gas at high pressure.
Furthermore, cleaning of the isolation valves of the fuel supply circuit is needed for the efficient operation of the gas turbine and of the fuel supply circuit. Thus, a leaking valve is likely to cause a stoppage of the gas turbine or a dangerous mix of fuel with other fluids present in the fuel supply circuit or in the purge circuit, even in the turbine control circuit.
Maintenance of the block and bleed valves is conducted through physical inspection and/or through pressurization tests, which are laborious and require decommissioning of the turbine during the time period of the inspection or tests. Although other methods permit testing the valves online, these methods require changing the turbine fuel supply circuit and installing branches or bypasses to ensure continuous supply of fuel to the combustion chambers.